32 xps studies the qualitative analysis of the

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3.2 XPS Studies The qualitative analysis of the coatings was performed to check FAS elements before the wettability experiments. Fig. 3a shows the broad XPS spectrum of the prepared superhydrophobic T-3 coating. Note that the obtained binding energies of 102.1, 284.5,
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Page 8 of 28 Accepted Manuscript 531.7, and 686 eV denote the existence of Si, C, O, and F, respectively. Indeed, the simplified formula of FAS is C 13 H 13 F 17 O 3 Si, that is, the measured material was confirmed as consisting of the components of FAS. Furthermore, to better analyze the hydrophobic fluoroalkyl group of FAS, high-resolution spectrum of C1s was observed ( Fig. 3e ) and C-C, CF 2 , and CF 3 bonds of the fluoroalkyl group were clearly shown at binding energies of 284.5, 290.5, and 292.6 eV, respectively [28]. The intensity of CF 2 peak is higher than C-C and CF 3 peaks because the 17 FTMS contains relatively more CF 2 molecules. 3.3 Wetting Properties of the Coatings The wettability of the sol-gel coatings prepared from different deposition times were depicted in Fig. 4 . A water contact angle gradually increases with deposition time of 10, 100 and 300 s, whereas decreased for 600 s of deposition time. For quick deposition time (10 s), T-1 coatings showed the water contact angle in Wenzel’s state having 127°. The water drop starts to move at a sliding angle of 31° showing its strong adhesion towards coating surface. This might be due to the less coverage of coating material and low surface roughness of the coatings. The air pocket trapping is futile and the hydrophobicity of the coatings is solely due to the nano-rough FAS sol-gel material. Generally, FAS coated smooth glass substrates show hydrophobic wetting state with water contact angle of 106° [29]. For 100 s of deposition time, water drop on T-2 coatings achieved contact angle of 141°, virtually achieving the spherical shape, however, the sliding angle value was still high of 19°. Even though, the surface roughness of the coating was slightly increased, the amount air pockets trapping in the rough structure is not sufficient and therefore water drop
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Page 9 of 28 Accepted Manuscript show adhesion towards coating surface despite of high contact angle. The hydrophobic Wenzel’s wetting state was switched to superhydrophobic Cassie-Baxter’s state for the deposition time of more than 100 s. The T-3 coatings prepared from 300 s of deposition time showed a water contact angle of 169° and the round-shaped water drops eventually rolled off the surface at a sliding angle of less than 5°. The increased surface roughness in conjunction with wrinkled hill-like morphology supports air pockets to be held in the rough interstices and so the superhydrophobic wetting state was achieved. A water drop placed on this coating surface may sit on the tip of hill-like structure having the trapped air pockets.
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  • Fall '19
  • The Land, Surface tension, Contact angle, superhydrophobic coatings

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